Analog Signal Isolator

ABSTRACT

Disclosed is a signal isolating test instrument, such as an electronics test probe. The instrument includes an input to receive a floating analog signal. An upconverter is employed to modulate the floating analog signal to a microwave frequency analog signal. An isolation barrier in the instrument prevents coupling of the floating analog signal to an earth ground. The instrument employs a microwave structure to transmit the microwave frequency analog signal across the isolation barrier via electromagnetic coupling. A downconverter is then employed to demodulate the microwave frequency analog signal to obtain a ground referenced test signal corresponding to the floating analog signal.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims benefit from U.S. Provisional PatentApplication Ser. No. 62/524295, filed Jun. 23, 2017, and entitled“Analog Signal Isolator,” which is incorporated herein by reference asif reproduced in its entirety.

TECHNICAL FIELD

This disclosure is directed to systems and methods associated withaspects of a test and measurement system, and, more particularly, tosystems and methods for providing isolation between floating signals andground referenced signals during signal testing.

BACKGROUND

Test and measurement systems are designed to receive signal inputs, forexample from a Device Under Test (DUT), sample the signals, and displaythe result as a waveform. A test and measurement system, such as anoscilloscope may contain a connection to ground. The oscilloscope mayalso measure voltages as a difference between an input voltage andground. However, in some cases, a user may wish to measure the voltageof a node relative to another node, both of which may float relative toground. In other cases, a DUT may be designed with a floating ground,such that the DUT employs components without a direct connection to theoscilloscope's ground. In such cases, a direct connection between theDUT and the oscilloscope results in measurements taken in reference tothe wrong ground, which may results in erroneous data, improper scaling,and may exceed the design constraints of the oscilloscope. For example,a pair of nodes operating at near one hundred Volts (V) would requirethe oscilloscope measure and display a hundred Volt value even if thedifference between the two signals is about one Volt. This may result inobfuscating fine differential measurements due to the large base voltagevalue. In order to overcome these concerns, isolation systems may beemployed to prevent signals in the DUT from becoming directly connectedto the oscilloscope's ground.

Examples in the disclosure address these and other issues.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects, features and advantages of embodiments of the presentdisclosure will become apparent from the following description ofembodiments in reference to the appended drawings in which:

FIG. 1 is a schematic diagram of an example test network including anisolation barrier between a floating signal and a ground referenced testand measurement system.

FIG. 2 is a schematic diagram of an example analog isolator.

FIG. 3 is a schematic diagram of an example waveguide based microwavestructure for an analog isolator.

FIG. 4 is a cross section of an example butt gap waveguide layout with aflange for a microwave structure.

FIG. 5 is a cross section of an example overlap gap waveguide layout fora microwave structure.

FIG. 6 is a cross section of an example butt gap waveguide layoutwithout a flange for a microwave structure.

FIG. 7 is a cross section of an example dielectric waveguide layout fora microwave structure.

FIGS. 8A-8C are schematic diagrams of an example coupled line basedmicrowave structure for an analog isolator.

FIG. 9 is a flowchart of an example method of transmitting a signal overan analog isolator via electromagnetic coupling.

FIG. 10 is a graph of an example signal coupling over an analog isolatoras a function of frequency.

DETAILED DESCRIPTION

Various isolation mechanisms may be employed to isolate floating signalsfrom a grounded test instrument. For example, a fiber optic system mayemploy the floating signal to modulate an optical signal at the DUT. Theoptical signal is then converted back to the electrical domain at thetest instrument. Such systems may require a separate power supply at theDUT, and hence may increase the complexity of the system. Further,optical technology may be expensive to implement. As another example, atransformer may magnetically couple the DUT to the test instrument whileproviding isolation. Such systems are limited to relatively lowfrequencies by parasitic capacitances within the transformer.

Disclosed herein is a mechanism for electromagnetically coupling afloating analog signal to a ground referenced test and measurementinstrument over an isolation barrier. The isolation barrier isimplemented between a DUT and a test and measurement system, for examplein a signal probe, a link, and/or a test and measurement system inputport/accessory. The mechanism employs an input to receive a floatinganalog signal and an upconverter to modulate the floating analog signalto a desired microwave frequency band (e.g. between about one Gigahertz(GHz) and about one hundred GHz). An isolation barrier is employed toisolate the DUT side of the circuit from ground. A microwave structureelectromagnetically couples the microwave frequency analog signal acrossthe isolation barrier, but does not couple signals outside of thedesired microwave frequency band (e.g. sub-microwave signals) across theisolation barrier. A downconverter then demodulates the microwavefrequency analog signal to obtain a ground referenced test signalcorresponding to the floating analog signal. An example microwavestructure may include a conductive waveguide with a gap at the isolationbarrier. The gap may include a dielectric with a high breakdown voltage,such as KAPTON. As another example, the microwave structure may includea dielectric waveguide selected to maintain a total internal reflectionat the microwave frequency band, and hence constrain the microwavefrequency analog signal in the waveguide while preventing movement ofout of band charge across the isolation barrier. As another example, themicrowave structure may include a pair of differential input conductivetraces electromagnetically coupled to a pair of differential outputconductive traces across the isolation barrier. The traces may bepositioned to share a virtual ground. The virtual ground may be an areawhere the voltage cancels out for the desired differential microwavesignal, and hence acts as a ground, while not canceling out common-modefrom the floating analog signal, and hence acts as an open circuit. Thedifferential conductive traces may include stub traces, which may act asbias connections for the upconverter/downconverter. The stub traces mayalso perform impedance matching functions, which may increase the rangeof the microwave frequency band that can couple across the isolationbarrier.

FIG. 1 is a schematic diagram of an example test network 100 includingan isolation barrier 125 between a floating signal and a groundreferenced test and measurement system. The test network 100 includes aDUT 110, a signal probe 113, a link 115, and a test system 117, whichmay be coupled as shown. A test signal 131 may be generated at the DUT110 and communicated to the test system 117 for testing.

A DUT 110 is any signal source configured to conduct electrical signals,such as a test signal 131. The DUT 110 may be any device that a user maydesire to test in order to determine relevant electricalcharacteristics. The test signal 131 may be any electrical signalgenerated by the DUT 110 and forwarded to the test system 117 fortesting. The DUT 110 may include a floating ground 121. A floatingground 121 describes any electrical circuit where a direct connection toan earth ground does not exist. A floating ground 121 may occur when auser desires to test the electrical characteristics of two nodesrelative to each other instead of relative to ground. A floating ground121 may also exist in battery powered devices that employ a chassis as alocal ground. A floating ground 121 may also exist when DUT componentsare grounded to a chassis as a local ground for noise reduction reasons.In any case, coupling a DUT 110 with a floating ground 121 may becomedangerous to a user and/or test equipment if not properly isolated.Specifically, when a grounded test system is fully coupled to such a DUT110, current in the DUT 110 (in addition to the test signal 131) mayseek the earth ground via the test system 117 and/or via the user. Thismay result in injury to the user and/or damage to the test equipment.

Test system 117 may be any system configured to test the electricalcharacteristics of the DUT 110. For example, test system 117 may includean oscilloscope with components to generate waveforms based on a testsignal 131, display such waveforms, perform frequency domain transforms,store calculated values based on the test signal 131, etc. The testsystem 117 may be ground referenced 123. A ground referenced 123 systemis any system with electronics that couple to an earth ground.

The test system 117 may couple to the DUT 110 via a signal probe 113and/or a link 115. A signal probe 113 is any device designed to coupleto the DUT 110 to provide a connection for the test signal 131. Thesignal probe 113 may contain attachments to secure the link 115 to anappropriate section of the DUT 110 to maintain contact with the testsignal 131. The link 115 may be any cable or other electricallyconductive material capable of conveying the test signal 131 to the testsystem 117.

As noted above, when a floating ground 121 is employed at a DUT 110, anisolation barrier 125 may be employed to prevent current in the DUT 110from seeking ground via the ground referenced 123 test system 117. Theisolation barrier 125 may include any non-conductive material that actsas an open circuit and hence denies current a path to ground via thetest system 117. The isolation barrier 125 may be implemented at thesignal output of the DUT 110, in the signal probe 113, in the link 115,and/or at an input port of the test system 117. As discussed below, amicrowave structure is employed to convey the test signal 131 across theisolation barrier 125 while denying Direct Current (DC) at the DUT 110 adirect path to ground across the isolation barrier 125. For purposes ofsimplicity, the isolation barrier 125 is generally discussed below asbeing implemented in a signal probe 113. However, the isolation barrier125 and attendant microwave structure may be implemented at any pointbetween a connection in the DUT 110 and the input of the test system 117(including at an input port of the test system 117).

FIG. 2 is a schematic diagram of an example analog isolator 200, whichmay be employed to traverse an isolation barrier in a test network, suchas test network 100. The analog isolator 200 may be implemented in asignal probe and/or in a test instrument input port. For example, theanalog isolator 200 circuitry may fit in a signal probe comp box. Theanalog isolator 200 includes an input 241, a preamplifier 242, anupconverter 243, an input oscillator 244, a microwave structure 250positioned across an isolation barrier 247, a downconverter 249, and anoutput oscillator 248 coupled as shown.

The input 241 is configured to receive a floating analog signal 232 atan input side 271 of the analog isolator 200. The implementation ofinput 241 may vary based on the implementation of the analog isolator200. For example, the input 241 may include a probe tip designed tocouple to a DUT. As another example, the analog isolator 200 may be anaccessory coupled between a probe and an oscilloscope, in which case theinput 241 may be a signal probe connector. The floating analog signal232 may be any test signal that is not coupled to an earth ground fortesting purposes. The floating analog signal 232 may be part of adifferential signal for which common mode voltage difference is to berejected during testing, a local chassis ground reference signal, and/orany other signal associated with a floating ground, such as floatingground 121, to be isolated from earth ground by the isolation barrier247.

The preamplifier 242 may be any amplifier configured to increase ordecrease the gain of the floating analog signal 232 so as to optimizethe signal amplitude for further processing. The preamplifier 242 mayalso provide a direct current (DC) offset to the floating analog signal232 to aid in optimizing the signal amplitude.

The upconverter 243 is coupled to the input 241 via the preamplifier242. The upconverter 243 is any component configured to increase afrequency of a signal. The upconverter 243 may also be referred to as amixer/upmixer. The upconverter 243 is designed to modulate the floatinganalog signal 232 into a microwave frequency analog signal 233.Specifically, the upconverter 243 increases the frequency of thefloating analog signal 232. As discussed below, the microwave structure250 is designed to block low frequency signals while allowing specifiedfrequency signals (e.g. microwave frequency signals) across theisolation barrier. Hence, the upconverter 243 increases the frequency ofthe floating analog signal 232 to place the signal into a band capableof passing through the microwave structure 250. The upconverter 243 isalso coupled to an input oscillator 244. The input oscillator 244 is anydevice capable of generating periodic oscillating electrical signals.The upconverter 243 employs the electrical signal from the inputoscillator 244 to increase the frequency of the floating analog signal232. For example, the upconverter 243 may multiply the floating analogsignal 232 with the electric signal from the input oscillator 244 toobtain the microwave frequency analog signal 233. As a specific example,a floating analog signal 232 of one GHz bandwidth and an inputoscillator 244 operating at six GHz may result in a microwave frequencyanalog signal 233 occupying a range of about five to seven GHz.

The isolation barrier 247 is designed to prevent coupling of thefloating analog signal 232 to an earth ground. For example, theisolation barrier 247 may be an area that does not include a conductivepath from an input side 271 of the analog isolator 200 (e.g. from theinput 241) to an output side 272 of the analog isolator 200. Hence, theisolation barrier 247 isolates the input side 271 from the output side272 by ensuring that electric charge has no direct conductive pathbetween the input side 271 and the output side 272. The isolationbarrier 247 may bisect the microwave structure 250. Also, the isolationbarrier 247 may be selected to be less than about one quarter wavelengthof the microwave frequency analog signal 233 across the microwavestructure 250 for reasons discussed below.

The microwave structure 250 is a circuit designed to transmit themicrowave frequency analog signal 233 across the isolation barrier 247via electromagnetic coupling. The microwave structure 250 includes atransmitting element and a receiving element separated by a gap ofnonconductive material. For example, the gap of nonconductive materialmay be selected as about one quarter of the wavelength or less of themicrowave frequency analog signal 233. This allows the microwavefrequency analog signal 233 to traverse the isolation barrier 247 whilepreventing sub-microwave signals from passing across the barrier 247.This allows the signal to cross while maintaining isolation andpreventing the floating analog signal 232 from becoming grounded. Asub-microwave signal may be any signal with a frequency below about oneGHz (e.g. including direct current and low frequency signals). Themicrowave structure 250 may act as an analog filter. Hence, themicrowave structure 250 may be tuned to allow desired frequency bandsacross (e.g. selected based on the input oscillator 244, for examplefive to seven GHz). Further, the upconverter 243 and input oscillator244 may be tuned to ensure that the microwave frequency analog signal233 occupies a frequency band that can be passed by the microwavestructure 250. For example, the microwave structure 250 may be designedto pass any signal within some range of the microwave band, which mayextend from between about one GHz and about one hundred GHz. As such,the upconverter 243 may increase the frequency of the floating analogsignal 232 into the microwave band, and the microwave structure 250 maypropagate the microwave frequency signal (e.g. microwave frequencyanalog signal 233) across the isolation barrier 247.

The analog isolator 200 includes a downconverter 249 on the output side272. The downconverter 249 is coupled to the microwave structure 250 andisolated from the floating analog signal 232 by the isolation barrier247. The downconverter 249 may also be referred to as a mixer/downmixer.The downconverter 249 is any component configured to decrease afrequency of a signal. The downconverter 249 is configured to demodulatethe microwave frequency analog signal 233 to obtain a ground referencedtest signal 234 corresponding to the floating analog signal 232. Inother words, the downconverter 249 is configured to reduce the frequencyto the same extent that the upconverter 243 increases the frequency.This ensures that the analog isolator 200 has a minimized effect on thesignal traversing the isolator 200. As such, the ground referenced testsignal 234 may be substantially the same signal as the floating analogsignal 232, but the input side 271 of the circuit may still be isolatedfrom ground connected to the output side 272 of the circuit. Thedownconverter 249 may be connected to an output oscillator 248, whichmay be substantially similar to the input isolator 244. The outputoscillator 248 may provide a periodic oscillating signal that isemployed (e.g. multiplied, mixed, etc.) by the downconverter 249 torestore the microwave frequency analog signal 233 to a frequency of thefloating analog signal 232. It should be noted that the input oscillator244 may be frequency locked with the output oscillator 248 across theisolation barrier 247. This allows the upconversion to be fullyreversible by the downconversion. The frequency lock between the inputoscillator 244 and output oscillator 248 may be achieved, for example bya microwave structure similar in design to microwave structure 250, atransformer, a capacitor, and/or an optical isolator.

FIG. 3 is a schematic diagram of an example waveguide based microwavestructure 300 for an analog isolator, such as analog isolator 200. Forexample, waveguide based microwave structure 300 may implement microwavestructure 250. Structure 300 is built on a substrate 357. The substrate357 may be any non-conductive structure to hold circuitry components,such as a Printed Circuit Board (PCB). The substrate 357 includes aninput side 371 and an output side 372, which may be substantiallysimilar to input side 271 and output side 272, respectively. Thesubstrate 357 also includes an isolation barrier 347 of non-conductivematerial positioned between the input side 371 and the output side 372.The isolation barrier 347 may be a subsection of a larger isolationbarrier traversing the entire device, such as isolation barrier 247.

The structure 300 may include an input waveguide launch 351 ofconductive material extending from the input side 371 of the substrate357 toward the isolation barrier 347. A waveguide launch, such as inputwaveguide launch 351, is any component that conductively transitions asignal from a link (e.g. coaxial line) to an electromagnetic wave forentry into a waveguide, such as a waveguide 353. The structure 300 mayalso include an output waveguide launch 355 of conductive materialextending from the output side 372 of the substrate 357 toward theisolation barrier 347. The output waveguide launch 355 may besubstantially similar to the input waveguide launch 351, but mayconductively transition an electromagnetic wave from the waveguide 353back to a link. For example, the input waveguide launch 351 and theoutput waveguide launch 355 may be made of sheet brass. The inputwaveguide launch 351 receives a microwave frequency analog signal 333 onthe input side 371. The microwave frequency analog signal 333 may besubstantially similar to microwave frequency analog signal 233. Themicrowave frequency analog signal 333 may be an upconverted floatingsignal as discussed above. The microwave frequency analog signal 333 isforwarded across the isolation barrier 347 via the waveguide 353 andforwarded off of the substrate 357 via the output waveguide launch 355on the output side 372 for downconversion as discussed above.

The structure 300 also includes the waveguide 353. The waveguide 353 iscoupled to the input waveguide launch 351 and the output waveguidelaunch 355. The waveguide 353 is positioned across the isolation barrier347. The waveguide 353 is structured to provide electro-magneticcoupling between the input waveguide launch 351 and the output waveguidelaunch 355 at micro-wave frequencies (e.g. one to one hundred GHz). Thewaveguide 353 is also structured to provide isolation between the inputwaveguide launch 351 and the output waveguide launch 355 when conductingdirect current. The waveguide 353 may employ various structures toprovide this functionality. For example, the waveguide 353 may beimplemented by employing one or more conductive structures on the inputside 371 and one or more corresponding conductive structures on theoutput side 372. Such conductive structures are separated at theisolation barrier 357 by a gap of non-conductive material and/or adielectric. Examples of such implementations are discussed with respectto FIGS. 4-7 below. Such a gap may be about one quarter wavelength ofthe microwave frequency analog signal 333 or less, which allowsmicrowave frequencies to cross the gap while providing isolation for DCsignals and hence preventing grounding of floating ground signals.

In another example, the waveguide 353 is made of a dielectric materialpositioned across the isolation barrier 347, and not made of conductivematerial. The dielectric is selected to provide total internalreflection at the boundaries of the dielectric to signals at microwavefrequencies, and hence constrain any coupled signal within thewaveguide. In other words, a dielectric waveguide 353 can propagate themicrowave frequency analog signal 333 across the isolation barrier 347via internal reflection. This approach provides isolation and propagatesthe signal. Further, this approach may couple the microwave frequencyanalog signal 333 farther than one quarter wavelength. This may allowthe structure 300 to be built with less required precision, and allowsfor a physically longer isolation break than a gapped conductor basedwaveguide. For example, in the absence of the conductor, a dielectricwaveguide 353 can provide arbitrarily high isolation voltage andarbitrarily low coupling capacitance by making the waveguide 353arbitrarily long.

FIGS. 4-7, as discussed below, depict various waveguides that may beemployed as part of waveguide 353 and/or as part of microwave structure250. In these examples, solid black represents a conductor (e.g. metal),and a bounded white box represents a dielectric. In such cases, thedielectric represents an isolation barrier. In example layouts 400, 500,and 600, microwave energy travels down an open space between upper andlower conductors, with the conductors serving to reflect microwaveenergy back into the waveguide that would otherwise spread outside. Inthe example layout 700, no conductors are employed, but total internalreflection at the dielectric's surfaces serves to guide the microwavesin a similar manner. The relative dimensions shown may be significant insome cases. For example, the open space between the waveguide walls mayinclude a dimension of about λ/2 to support the wave propagation,whereas the gaps/dielectrics/isolators may include a dimension of lessthan λ/4 to minimize energy leakage through the gap. In this context λis the wavelength of the microwave signal to be propagated over thegap/dielectric/isolator. It should be noted that the dielectricwaveguide is depicted with a smaller dimension because the wavelengthfor a specified frequency over a dielectric is shorter than in thewavelength for the same frequency signal in air. Also, the surface of adielectric may pick up some environmental contamination duringmanufacture, and is hence likely to withstand less electric field than asolid dielectric. As such, the dielectric placement in each of layouts400, 500, and 600 allows for a longer isolation distance along thedielectric surface than through the bulk of the dielectric. The gapbetween the conductors may be small to minimize microwave leakage, butthe dielectric may extend beyond the gap to increase the surfaceisolation distance.

FIG. 4 is a cross section of an example butt gap waveguide layout 400with a flange for a microwave structure, such as microwave structure250. For example, layout 400 may be employed to construct a waveguide353. Butt gap waveguide layout 400 is an example waveguide with a gapacross an isolation barrier 447. The waveguide layout 400 includes inputwaveguide material 461 and output waveguide material 463 with a gap atthe isolation barrier 447. In layout 400, the input waveguide material461 and output waveguide material 463 each include a hollow portion(e.g. filled with air) enclosed by metal walls. A signal traversing thewaveguide material 461 and/or 463 may traverse the hollow area andbounce off of the walls due to internal reflection.

The gap may be referred to as a butt gap as the ends of the waveguidematerials 461 and 463 generally align to butt up to the isolationbarrier 447 against each other with a gap in between. This may besubstantially similar to isolation barriers 147, 247, and/or 347. Theinput waveguide material 461 and the output waveguide material 463 maybe electromagnetically coupled across the gap at the isolation barrier447. Hence, a microwave frequency analog signal 433 may traverse theinput waveguide material 461 and traverse the isolation barrier via theelectromagnetic coupling between the input waveguide material 461 andthe output waveguide material 463. The microwave frequency analog signal433 may be substantially similar to microwave frequency analog signals233 and/or 333.

The gap is shown in FIG. 4 as an isolator 465. The isolator 465 is anycomponent that prevents direct current flow and hence prevents groundingof a signal on the input side of the layout 400. In some example, theisolator 465 may include an area of PCB without a conductive trace (e.g.without a conductive wall). This may also be referred to as an air gap.In another example, the isolator 465 may include a dielectric materialacross the isolation barrier 447. Employing a dielectric as an isolator465 provides a structural component for other components to couple to,and hence provides extra stability as well as isolation. As such, adielectric included in the isolator 465 may allow for less precisemanufacturing processes. A dielectric with a high breakdown voltage,such as KAPTON, may be employed. The approach can provide multiplekilovolts (kVs) of galvanic isolation with gaps on the order of fivemillimeters. As noted above, the isolator 465 may be less than about onequarter wavelength of the microwave frequency analog signal 433.

FIG. 5 is a cross section of an example overlap gap waveguide layout 500for a microwave structure, such as microwave structure 250. For example,layout 500 may be employed to construct a waveguide 353 in a mannersimilar to layout 400. Layout 500 includes input waveguide material 561,output waveguide material 563, an isolation barrier 547, and an isolator565, which may be substantially similar to the input waveguide material461, the output waveguide material 463, the isolation barrier 447, andthe isolator 465, respectively. A microwave frequency analog signal 533,which may be substantially similar to the microwave frequency analogsignal 433 travels through the layout 500. However, the waveguide layout500 includes waveguide material 561 and 563 with an overlap gap at theisolation barrier 547. The input waveguide material 561 and outputwaveguide material 563 may extend to the isolation barrier 547. In someexamples, the input waveguide material 561 and output waveguide material563 may even extend over the isolation barrier 547. However, theisolator 565 maintains the isolation barrier 547, even when such overlapoccurs. The overlap gap waveguide layout 500 shown may have certainbeneficial properties when performing electromagnetic coupling. Forexample, electromagnetic coupling in an overlap gap configuration mayresult in very little signal leakage due to the physical structure ofthe conductive material. As with layout 400, the input waveguidematerial 561 and output waveguide material 563 each include a hollowportion (e.g. filled with air) enclosed by metal walls. A signaltraversing the waveguide material 561 and/or 563 may traverse the hollowarea and bounce off of the walls due to internal reflection, in thiscase without traversing the isolator 565.

FIG. 6 is a cross section of an example butt gap waveguide layout 600without a flange for a microwave structure, such as microwave structure250. For example, layout 600 may be employed to construct a waveguide353 in a manner similar to layouts 400 and 500. Layout 600 includesinput waveguide material 661, output waveguide material 663, anisolation barrier 647, and isolator 665, which may be substantiallysimilar to the input waveguide material 461, output waveguide material463, isolation barrier 447, and isolator 465, respectively. A microwavefrequency analog signal 633, which may be substantially similar to themicrowave frequency analog signal 433 travels through the layout 600.Unlike layout 400, layout 600 employs an isolator 665 between the inputwaveguide material 661 and the output waveguide material 663. In layout400, the input waveguide material 661 and output waveguide material 663each include a hollow portion (e.g. filled with air) enclosed by metalwalls. In this case, the hollow portion extends across the isolationbarrier 647. As such, the microwave frequency analog signal 633traverses the hollow portion without traversing the isolator 665.

FIG. 7 is a cross section of an example dielectric waveguide layout 700for a microwave structure, such as microwave structure 250. For example,layout 700 may be employed to construct a waveguide 353 in a mannersimilar to layouts 400, 500, and 600. Layout 700 includes an isolator765 that traverses an isolation barrier 747. In this case, an inputwaveguide material 761 and an output waveguide material 763 include theisolator 765. An microwave frequency analog signal 733 may traverse theisolator 765. The isolator 765 includes non-conductive material andincludes a length selected to allow the microwave frequency analogsignal 733 to extend across the isolator 765 while preventingsub-microwave frequency signals from crossing.

FIGS. 8A-8C are schematic diagrams of an example coupled line basedmicrowave structure 800 for an analog isolator, such as analog isolator200. For example, the coupled line based microwave structure 800 mayimplement microwave structure 250. FIGS. 8A, 8B, and 8C illustrate a topview, a bottom view, and a cross sectional view, respectively, of thecoupled line based microwave structure 800. The structure 800 is builton a substrate 857, such as a PCB. For purposes of discussion, thesubstrate 857 includes an input side 871 as shown in FIG. 8A and anoutput side 872 as shown in FIG. 8B. The substrate 857 also includes aproximate end 873 and a distal end 874. The coupled line based microwavestructure 800 illustrates an example microwave structure that includesan input conductor on an input side of an isolation barrier, the inputconductor implemented as differential input traces. The structure 800also includes an output conductor on an output side of the isolationbarrier implemented as differential output traces. The input conductorand the output conductor are then electromagnetically coupled across theisolation barrier. Reference is first made to the input side 871 asshown in FIG. 8A.

Structure 800 is implemented as a differential system. In a differentialsystem, a propagated signal value is represented as a difference betweena pair of signal values. Hence, the structure 800 includes a pair ofdifferential input traces extending from the proximate end 873 of theinput side 871 of the substrate 857. The pair of differential inputtraces includes a positive input trace 854 and a negative input trace855. The positive input trace 854 is coupled to a positive input 852 andthe negative input trace 855 is coupled to a negative input 851,respectively. For example, the positive input 852 may receive a positivefloating analog signal from a positive output of an upconverter.Likewise, the negative input 851 may receive a negative floating analogsignal from a negative output of an upconverter.

The positive input trace 854 and the negative input trace 855 includeconductive material and extend from the proximate end 873 toward anisolation barrier 847, which may be substantially similar to isolationbarrier 247. As such, the positive input trace 854 conducts a positivefloating analog signal and the negative input trace 855 conducts anegative floating analog signal, respectively. Referring to FIG. 8C, theisolation barrier 847 includes non-conductive material (e.g. PCBmaterial). The isolation barrier 847 is included in the substrate 857and extends between the input side 871 and the output side 872. Thepositive input trace 854 and negative input trace 855 are applied to theinput side 871 of the substrate 857 over the isolation barrier 847.Referring back to FIG. 8A., the positive input trace 854 and negativeinput trace 855 carry complementary and opposite signals. As such, thevoltage induced between the positive input trace 854 and the negativeinput trace 855 cancels out in an area between the traces. This resultsin an area of zero voltage, which acts as a virtual common ground 861for differential signals. The virtual common ground 861 acts as a groundfrom an electrical standpoint, but is not conductive and hence does notact as a ground connection relative to the isolation barrier 847 fordirect current voltages.

Referring now to FIG. 8B, the output side 872 substantially mirrors theinput side 871. The structure 800 includes a pair of differential outputtraces extending from the distal end 874 of the output side 872 of thesubstrate 857. The differential output traces include a positive outputtrace 858 to conduct a positive ground referenced test signalcorresponding to the positive floating analog signal. The differentialoutput traces also include a negative output trace 856 to conduct anegative ground referenced test signal corresponding to the negativefloating analog signal. The positive output trace 858 is coupled to apositive output 863 and the negative output trace 856 is coupled to anegative output 862, respectively. For example, the positive output 863may output a positive floating analog signal to a positive input of adownconverter. Likewise, the negative output 862 may output a negativefloating analog signal to a negative input of a downconverter. As notedabove, the isolation barrier 847 is positioned between the differentialinput traces and the differential output traces. The differential inputtraces and the differential output traces electromagnetically coupleacross the isolation barrier at micro-wave frequencies. For example, thepositive input trace 854 transmits the positive floating analog signalacross the isolation barrier 847 for receipt at the positive outputtrace 858 as a positive ground referenced test signal. Likewise, thenegative input trace 855 transmits the negative floating analog signalacross the isolation barrier 847 for receipt at the negative outputtrace 856 as a negative ground referenced test signal. However, theisolation barrier 847 is non-conductive. As such, the isolation barrier847 provides isolation of the differential input traces from thedifferential output traces against conducting direct current.Accordingly, the isolation barrier 847 prevents the floating analogsignals on the input side from becoming grounded, while allowingmicrowave frequency signals to cross (e.g. after upconversion asdiscussed with respect to FIG. 2). Further, the positive output trace858 and the negative output trace 856 are positioned to create a virtualground 861 in the same manner as the input traces. As such, thedifferential input traces and differential output traces are positionedto create a virtual common ground 861 plane for the differentialmicrowave analog signals and a virtual open for the common-mode signalfrom the floating input. In other words, the input conductor includes apair of differential conductors sharing a virtual ground, and the outputconductor includes a pair of differential conductors sharing the samevirtual ground. Thus positive input trace 854 and positive output trace858 form a coupled transmission line bandpass filter over the commonvirtual ground, and may be tuned to couple the microwave analog signalacross the isolation barrier. Similarly, negative input trace 855 andnegative output trace 856 form a coupled transmission line bandpassfilter over the common virtual ground, and may be similarly tuned tocouple the microwave analog signal across the isolation barrier.

Referring to FIG. 8C, the substrate 857 contains a conductive layeracting as a floating ground 865 on the proximate end. The substrate 857also includes a conductive layer acting as an earth ground 864 on thedistal end. The floating ground 865 and the earth ground 864 areseparated by the isolation barrier 847 to maintain isolation between theinput side 871 and the output side 872. Grounds 865 and 864 are employedby various components to complete the conductive circuits. For example,a completed circuit from an upconverter 243 may require a path to afloating ground, while a completed circuit from a downconverter 249 mayrequire a path to an earth ground. Floating ground 865 and earth ground864 may provide such ground paths. Input stub traces 853, as shown inFIG. 8A, may provide a connection to the floating ground 865 from thedifferential input traces. Also, output stub traces 859, as shown inFIG. 8B, may provide a connection to earth ground 864 from thedifferential output traces. The stub traces 853 and 859 may be made ofconductive material. Further, the stub traces 853 and 859 may be sizedat a quarter wavelength in length between the isolation barrier 847 anda point where the corresponding stub trace connects through a substratelayer to a corresponding ground 865 and 864, respectively. Thedifferential input traces and differential output traces include quarterwavelength stub traces 853 and 859 to provide impedance matching atcarrier frequencies and local ground shorts outside of carrierfrequencies. For example, quarter-wave stubs may provide high impedanceat the carrier frequency, but may drop in impedance as the frequencydeviates from the carrier and may become shorts at DC or twice thecarrier frequency. The coupled-line bandpass filter, however, mayprovide increased impedance as the frequency deviates from the carrier,and may become open at DC or twice the carrier frequency. The twoimpedance shifts work to partially cancel each other, allowing a betterimpedance match over a wider bandwidth.

As a specific example, at an impedance of fifty Ohms per side, withdifferential impedance of about one hundred Ohms for both sides, eachcoupled line pair may have an even-mode impedance that is one hundredOhms greater than its odd-mode impedance, both taken with respect to thevirtual ground plane extending across the virtual common ground 861. Thestub traces 853 and 859 may act as a filter for the microwave frequencysignals traversing the isolation barrier 847. For example, the stubtraces 853 and 859, when sized to about a quarter wavelength, mayincrease the size of the microwave frequency band that can traverse theisolation barrier. Hence, the stub traces 853 and 859 may act as awideband filter, and may increase the effective microwave frequencyrange of the isolator. In other words, the microwave structure 800includes a stub 853 operating as a DC path for the upconverter and as animpedance matching filter for a microwave frequency analog signal. Also,the microwave structure 800 includes a stub 859 operating as a DC pathfor the downconverter and as an impedance matching filter for themicrowave frequency analog signal. It should be noted that, in someexamples, the stub traces 853 and 859 may be omitted in favor of a dualof the abovementioned differential input traces and differential outputtraces with unconnected ends of the coupled lines shorted to ground.Such a configuration may function in a similar manner, but may bedifficult to implement due to geometric constraints.

FIG. 9 is a flowchart of an example method 900 of transmitting a signalover an analog isolator via electromagnetic coupling. For example,method 900 may be implemented in an analog isolator 200 operating in atest network 100. Method 900 may operate on a microwave structure 300employing any of waveguide layouts 400, 500, 600, 700. Method 900 mayalso operate on a coupled line based microwave structure 800. At block901, a floating analog signal is received at an input, such as input241. The floating analog signal is then preamplified at block 903 by apreamplifier. The floating analog signal is amplified in order tooptimize the signal amplitude for the dynamic range of subsequentcircuitry, such as an upconverter and/or downconverter. At block 905,the floating analog signal is modulated into a microwave frequencyanalog signal. For example, an upconverter 243 may increase a frequencyof the floating analog signal by mixing/convolving the floating analogsignal with an oscillating signal from an input oscillator. At block907, the microwave frequency analog signal is transmitted across anisolation barrier via a microwave structure, such as microwave structure300 and/or 800. At block 909, the microwave frequency analog signal maybe demodulated to obtain a ground referenced test signal correspondingto the floating analog signal. For example, a downconverter 249 maydecrease the frequency of the microwave frequency analog signal bymixing/convolving the microwave frequency analog signal with anoscillating signal from an output oscillator 248. This results in aground referenced test signal that is substantially similar to, andcorresponds to, a floating analog signal while maintaining an isolationbarrier preventing grounding between devices.

FIG. 10 is a graph 1000 of an example signal coupling over an analogisolator, such as an analog isolator 200, as a function of frequency.Graph 1000 depicts voltage gain at the isolation barrier in units ofdecibels (dB) versus signal frequency in units of gigahertz (GHz). Asshown, below about one GHz there is very little transmission. As thefrequency increases into the microwave range (e.g. above one GHz), thegain approaches 0 dB or full transmission. For the example shown, themajority of power is transmitted across the barrier between about twoGHz and about six GHz. This supports two GHz of modulation on eitherside of a four GHz midpoint. However, it should be noted that tuning(e.g. adjusting the physical dimensions of the circuit configuration)may alter the frequency band that is allowed to pass the isolationfilter. The various analog isolators discussed herein may operate insubstantially any portion of the microwave frequency range.

Examples of the disclosure may operate on a particularly createdhardware, on firmware, digital signal processors, or on a speciallyprogrammed general purpose computer including a processor operatingaccording to programmed instructions. The terms “controller” or“processor” as used herein are intended to include microprocessors,microcomputers, ASICs, and dedicated hardware controllers. One or moreaspects of the disclosure may be embodied in computer-usable data andcomputer-executable instructions, such as in one or more programmodules, executed by one or more computers (including monitoringmodules), or other devices. Generally, program modules include routines,programs, objects, components, data structures, etc. that performparticular tasks or implement particular abstract data types whenexecuted by a processor in a computer or other device. The computerexecutable instructions may be stored on a non-transitory computerreadable medium such as a hard disk, optical disk, removable storagemedia, solid state memory, RAM, etc. As will be appreciated by one ofskill in the art, the functionality of the program modules may becombined or distributed as desired in various examples. In addition, thefunctionality may be embodied in whole or in part in firmware orhardware equivalents such as integrated circuits, field programmablegate arrays (FPGA), and the like. Particular data structures may be usedto more effectively implement one or more aspects of the disclosure, andsuch data structures are contemplated within the scope of computerexecutable instructions and computer-usable data described herein.

Aspects of the present disclosure operate with various modifications andin alternative forms. Specific aspects have been shown by way of examplein the drawings and are described in detail herein below. However, itshould be noted that the examples disclosed herein are presented for thepurposes of clarity of discussion and are not intended to limit thescope of the general concepts disclosed to the specific examplesdescribed herein unless expressly limited. As such, the presentdisclosure is intended to cover all modifications, equivalents, andalternatives of the described aspects in light of the attached drawingsand claims.

References in the specification to embodiment, aspect, example, etc.,indicate that the described item may include a particular feature,structure, or characteristic. However, every disclosed aspect may or maynot necessarily include that particular feature, structure, orcharacteristic. Moreover, such phrases are not necessarily referring tothe same aspect unless specifically noted. Further, when a particularfeature, structure, or characteristic is described in connection with aparticular aspect, such feature, structure, or characteristic can beemployed in connection with another disclosed aspect whether or not suchfeature is explicitly described in conjunction with such other disclosedaspect.

The disclosed aspects may be implemented, in some cases, in hardware,firmware, software, or any combination thereof. The disclosed aspectsmay also be implemented as instructions carried by or stored on one ormore or non-transitory computer-readable media, which may be read andexecuted by one or more processors. Such instructions may be referred toas a computer program product. Computer-readable media, as discussedherein, means any media that can be accessed by a computing device. Byway of example, and not limitation, computer-readable media may comprisecomputer storage media and communication media.

Computer storage media means any medium that can be used to storecomputer-readable information. By way of example, and not limitation,computer storage media may include Random Access Memory (RAM), Read OnlyMemory (ROM), Electrically Erasable Programmable Read-Only Memory(EEPROM), flash memory or other memory technology, Compact Disc ReadOnly Memory (CD-ROM), Digital Video Disc (DVD), or other optical diskstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, and any other volatile or nonvolatile,removable or non-removable media implemented in any technology. Computerstorage media excludes signals per se and transitory forms of signaltransmission.

Communication media means any media that can be used for thecommunication of computer-readable information. By way of example, andnot limitation, communication media may include coaxial cables,fiber-optic cables, air, or any other media suitable for thecommunication of electrical, optical, Radio Frequency (RF), infrared,acoustic or other types of signals.

EXAMPLES

Illustrative examples of the technologies disclosed herein are providedbelow. An embodiment of the technologies may include any one or more,and any combination of, the examples described below.

Example 1 includes an apparatus comprising: an input to receive afloating analog signal; an upconverter, coupled to the input, theupconverter to modulate the floating analog signal to a microwavefrequency analog signal; an isolation barrier to prevent coupling of thefloating analog signal to an earth ground; and a microwave structure totransmit the microwave frequency analog signal across the isolationbarrier via electromagnetic coupling.

Example 2 includes the apparatus of Example 1, further comprising adownconverter coupled to the microwave structure and isolated from thefloating analog signal by the isolation barrier, the downconverter todemodulate the microwave frequency analog signal to obtain a groundreferenced test signal corresponding to the floating analog signal.

Example 3 includes the apparatus of Example 2, wherein the microwavestructure couples a majority of the microwave frequency analog signalfrom the upconverter to the downconverter.

Example 4 includes the apparatus of Examples 1-3, wherein the microwavestructure includes a waveguide with a gap across the isolation barrier.

Example 5 includes the apparatus of Example 4, wherein the gap acrossthe isolation barrier includes a dielectric material.

Example 6 includes the apparatus of Examples 1-5, wherein the microwavestructure includes a dielectric waveguide to propagate the microwavefrequency analog signal across the isolation barrier via internalreflection.

Example 7 includes the apparatus of Examples 1-6, wherein the microwavestructure includes an input conductor on an input side of the isolationbarrier and an output conductor on an output side of the isolationbarrier, the input conductor and the output conductorelectromagnetically coupled across the isolation barrier.

Example 8 includes the apparatus of Example 7, wherein the inputconductor includes a pair of differential conductors sharing a virtualground, and the output conductor includes a pair of differentialconductors sharing a virtual ground.

Example 9 includes the apparatus of Examples 6-8, wherein the microwavestructure includes a stub operating as a DC path for the upconverter andas an impedance matching filter for the microwave frequency analogsignal.

Example 10 includes the apparatus of Examples 1-9, wherein isolationbarrier bisects the microwave structure, and the isolation barrier isless than one quarter wavelength of the microwave frequency analogsignal.

Example 11 includes an apparatus comprising: a substrate including aninput side, an output side, a proximate end, and a distal end; a pair ofdifferential input traces extending from the proximate end of the inputside of the substrate; a pair of differential output traces extendingfrom the distal end of the output side of the substrate; an isolationbarrier of non-conductive material included in the substrate, theisolation barrier positioned between the differential input traces andthe differential output traces for electro-magnetic coupling between thedifferential input traces and the differential output traces atmicro-wave frequencies and isolation of the differential input tracesfrom the differential output at sub-microwave frequencies.

Example 12 includes the apparatus of Example 11, wherein thedifferential input traces include: a positive input trace to conduct apositive floating analog signal; and a negative input trace to conduct anegative floating analog signal.

Example 13 includes the apparatus of Examples 11-12, wherein thedifferential output traces include: a positive output trace to conduct apositive ground referenced test signal corresponding to the positivefloating analog signal; and a negative output trace to conduct anegative ground referenced test signal corresponding to the negativefloating analog signal.

Example 14 includes the apparatus of Examples 11-13, wherein thedifferential input traces and differential output traces are positionedto create a virtual common ground plane for differential signals and avirtual common open for common-mode signals.

Example 15 includes the apparatus of Examples 11-14, wherein thedifferential input traces include quarter wavelength stub tracesproviding impedance matching at micro-wave frequencies and local biascurrent paths.

Example 16 includes an apparatus comprising: a substrate including aninput side and an output side; an isolation barrier of non-conductivematerial positioned between the input side and the output side; an inputwaveguide launch of conductive material extending from the input sidetoward the isolation barrier; an output waveguide launch of conductivematerial extending from the output side toward the isolation barrier;and a waveguide coupled to the input waveguide launch and outputwaveguide launch, the waveguide positioned across the isolation barrierfor electro-magnetic coupling between the input waveguide launch and theoutput waveguide launch at micro-wave frequencies and isolation betweenthe input waveguide launch and the output waveguide launch atsub-microwave frequencies.

Example 17 includes the apparatus of Example 16, wherein the waveguideincludes a dielectric to constrain coupled signals within the waveguidedue to total internal reflection at boundaries of the dielectric.

Example 18 includes the apparatus of Example 16, wherein the waveguideincludes waveguide material with an overlap gap at the isolationbarrier.

Example 19 includes the apparatus of Example 16, wherein the waveguideincludes waveguide material with a butt gap at the isolation barrier.

Example 20 includes the apparatus of Examples 16-19, wherein thewaveguide includes waveguide material with a gap at the isolationbarrier, the gap including a dielectric.

Example 21 includes a method comprising: receiving a floating analogsignal; modulating the floating analog signal into a microwave frequencyanalog signal; and transmitting the microwave frequency analog signalacross an isolation barrier in a test and measurement system via amicrowave structure.

Example 22 includes the method of Example 21, further comprisingpreamplifying the floating analog signal prior to modulating thefloating analog signal.

Example 23 includes the method of Example 21, further comprisingdemodulating the microwave frequency analog signal to obtain a groundreferenced test signal corresponding to the floating analog signal.

Example 24 includes the method of Example 23, wherein the floatinganalog signal is modulated into the microwave frequency analog signal byconvolving the floating analog signal with an oscillating signal from aninput oscillator, wherein the microwave frequency analog signal isdemodulated by convolving the microwave frequency analog signal with anoscillating signal from an output oscillator, and wherein the outputoscillator is frequency locked to the input oscillator.

Example 25 includes the method of Examples 21-24, wherein the isolationbarrier bisects the microwave structure, and the isolation barrier isless than one quarter wavelength of the microwave frequency analogsignal.

Example 26 includes the method of Examples 21-25, wherein the isolationbarrier bisects the microwave structure, and the microwave frequencyanalog signal is constrained in the microwave structure while traversingthe isolation barrier due to total internal reflection of the microwavestructure.

The previously described examples of the disclosed subject matter havemany advantages that were either described or would be apparent to aperson of ordinary skill. Even so, all of these advantages or featuresare not required in all versions of the disclosed apparatus, systems, ormethods.

Additionally, this written description makes reference to particularfeatures. It is to be understood that the disclosure in thisspecification includes all possible combinations of those particularfeatures. Where a particular feature is disclosed in the context of aparticular aspect or example, that feature can also be used, to theextent possible, in the context of other aspects and examples.

Also, when reference is made in this application to a method having twoor more defined steps or operations, the defined steps or operations canbe carried out in any order or simultaneously, unless the contextexcludes those possibilities.

Although specific examples of the disclosure have been illustrated anddescribed for purposes of illustration, it will be understood thatvarious modifications may be made without departing from the spirit andscope of the disclosure. Accordingly, the disclosure should not belimited except as by the appended claims.

We claim:
 1. An apparatus comprising: an input to receive a floatinganalog signal; an upconverter, coupled to the input, the upconverter tomodulate the floating analog signal to a microwave frequency analogsignal; an isolation barrier to prevent coupling of the floating analogsignal to an earth ground; and a microwave structure to transmit themicrowave frequency analog signal across the isolation barrier viaelectromagnetic coupling.
 2. The apparatus of claim 1, furthercomprising a downconverter coupled to the microwave structure andisolated from the floating analog signal by the isolation barrier, thedownconverter to demodulate the microwave frequency analog signal toobtain a ground referenced test signal corresponding to the floatinganalog signal.
 3. The apparatus of claim 2, wherein the microwavestructure couples a majority of the microwave frequency analog signalfrom the upconverter to the downconverter.
 4. The apparatus of claim 1,wherein the microwave structure includes a waveguide with a gap acrossthe isolation barrier.
 5. The apparatus of claim 4, wherein the gapacross the isolation barrier includes a dielectric material.
 6. Theapparatus of claim 1, wherein the microwave structure includes adielectric waveguide to propagate the microwave frequency analog signalacross the isolation barrier via internal reflection.
 7. The apparatusof claim 1, wherein the microwave structure includes an input conductoron an input side of the isolation barrier and an output conductor on anoutput side of the isolation barrier, the input conductor and the outputconductor electromagnetically coupled across the isolation barrier. 8.The apparatus of claim 7, wherein the input conductor includes a pair ofdifferential conductors sharing a virtual ground and the outputconductor includes a pair of differential conductors sharing a virtualground.
 9. The apparatus of claim 7, wherein the microwave structureincludes a stub operating as a direct current (DC) path for theupconverter and as an impedance matching filter for the microwavefrequency analog signal.
 10. The apparatus of claim 1, wherein isolationbarrier bisects the microwave structure, and the isolation barrier isless than one quarter wavelength of the microwave frequency analogsignal.
 11. An apparatus comprising: a substrate including an inputside, an output side, a proximate end, and a distal end; a pair ofdifferential input traces extending from the proximate end of the inputside of the substrate; a pair of differential output traces extendingfrom the distal end of the output side of the substrate; an isolationbarrier of non-conductive material included in the substrate, theisolation barrier positioned between the differential input traces andthe differential output traces for electro-magnetic coupling between thedifferential input traces and the differential output traces atmicro-wave frequencies and isolation of the differential input tracesfrom the differential output traces at sub-microwave frequencies. 12.The apparatus of claim 11, wherein the differential input tracesinclude: a positive input trace to conduct a positive floating analogsignal; and a negative input trace to conduct a negative floating analogsignal.
 13. The apparatus of claim 12, wherein the differential outputtraces include: a positive output trace to conduct a positive groundreferenced test signal corresponding to the positive floating analogsignal; and a negative output trace to conduct a negative groundreferenced test signal corresponding to the negative floating analogsignal.
 14. The apparatus of claim 11, wherein the differential inputtraces and differential output traces are positioned to create a virtualcommon ground plane for differential signals and a virtual common openfor common-mode signals.
 15. The apparatus of claim 11, wherein thedifferential input traces include quarter wavelength stub tracesproviding impedance matching at micro-wave frequencies and local biascurrent paths.
 16. An apparatus comprising: a substrate including aninput side and an output side; an isolation barrier of non-conductivematerial positioned between the input side and the output side; an inputwaveguide launch of conductive material extending from the input sidetoward the isolation barrier; an output waveguide launch of conductivematerial extending from the output side toward the isolation barrier;and a waveguide coupled to the input waveguide launch and outputwaveguide launch, the waveguide positioned across the isolation barrierfor electro-magnetic coupling between the input waveguide launch and theoutput waveguide launch at micro-wave frequencies and isolation betweenthe input waveguide launch and the output waveguide launch atsub-microwave frequencies.
 17. The apparatus of claim 16, wherein thewaveguide includes a dielectric to constrain coupled signals within thewaveguide due to total internal reflection at boundaries of thedielectric.
 18. The apparatus of claim 16, wherein the waveguideincludes waveguide material with an overlap gap at the isolationbarrier.
 19. The apparatus of claim 16, wherein the waveguide includeswaveguide material with a butt gap at the isolation barrier.
 20. Theapparatus of claim 16, wherein the waveguide includes waveguide materialwith a gap at the isolation barrier, the gap including a dielectric. 21.A method comprising: receiving a floating analog signal; modulating thefloating analog signal into a microwave frequency analog signal; andtransmitting the microwave frequency analog signal across an isolationbarrier in a test and measurement system via a microwave structure. 22.The method of claim 21, further comprising preamplifying the floatinganalog signal prior to modulating the floating analog signal.
 23. Themethod of claim 21, further comprising demodulating the microwavefrequency analog signal to obtain a ground referenced test signalcorresponding to the floating analog signal.
 24. The method of claim 23,wherein the floating analog signal is modulated into the microwavefrequency analog signal by convolving the floating analog signal with anoscillating signal from an input oscillator; the microwave frequencyanalog signal is demodulated by convolving the microwave frequencyanalog signal with an oscillating signal from an output oscillator; andthe output oscillator is frequency locked to the input oscillator. 25.The method of claim 21, wherein the isolation barrier bisects themicrowave structure, and the isolation barrier is less than one quarterwavelength of the microwave frequency analog signal.
 26. The method ofclaim 21, wherein the isolation barrier bisects the microwave structure,and the microwave frequency analog signal is constrained in themicrowave structure while traversing the isolation barrier due to totalinternal reflection of the microwave structure.